Dark Matter

There is nowadays compelling evidence that most of the Universe is filled by an unconventional, invisible matter and energy. The most recent cosmological observations point to a 74% of Dark Energy, and 22% of Dark Matter, leaving less than 5% for the conventional matter (planets, galaxies, intergalactic dust, ourselves,..). Most of our Universe is dark and mysterious!

We know practically nothing about the Dark Energy, and very little about Dark Matter. It seems that the Dark Matter could be made of particles with certain known properties, but that they are none of the standard particles that physicists study at particle accelerators like CERN (i.e. the well-known repertoire of the “Standard Model of Particle Physics”). Therefore, looking for the Dark Matter is searching for new particles “beyond the Standard Model”, i.e. “New Physics”.

Although there no strong experimental evidence that the Standard Model needs to be extended (other than the existence of the Dark Matter!), there are arguments of theoretical or fundamental character for expecting so. Indeed, theoretical physicists have proposed a number of possible extensions of the Standard Model, some of them include new particles that could compose the Dark Matter.

WIMP stands for “weakly interacting massive particle”. This generic type of particles appear in several of the mentioned extensions, like in the popular Supersymmetry. WIMPs are relatively heavy and neutral. They look like “big neutrons”. If WIMPs exist, they have the right properties to form the Dark Matter, that is, they would have been produced soon after the Big Bang, and they would still be left around in sufficient quantities. Another popular candidate are the axions, proposed to solve the so-called strong CP problem. Axions, unlike WIMPs, are very light particles (but not massles) that would be coupled with the photons. They would look like “strange photons”. Despite their low mass, they could be copiously produced soon after the Big Bang thanks to special mechanisms, and therefore they could perfectly be the Dark Matter.

Depending on whether the Dark Matter is made of WIMPs or axions, its detection might be possible with sufficiently sensitive experiments of one or other kind. Dark Matter WIMPs could be detected via their interaction with nuclei, producing tiny -but observable- nuclear recoils. In order to measure WIMP-induced nuclear recoils, one needs to build large sensitive masses, extremely low levels of background, and strategies to identify, as much as possible, “signal” events from background ones. As part of this strategies, this experiments are build in Underground LAboratories, like the Laboratorio Subterráneo de Canfranc.

Axions can also be detected with completely different type of detectors.